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Proton Transfer Reactions of Nickel ( 11)-Triglycine E. J. Billo and Dale W. Margeruml Contribution from the Department of Chemistry, Purdue University, Lafayette, Indiana 47097. Received January 17, 1970 Abstract: The reaction of NiH-2L - (where L- is the triglycinate ion and two protons are ionized from the peptide nitrogens) with H30+is 1W times slower than the diffusion-controlled limit. Other acids also are slow to react with NiH-,L- and follow a general acid catalysis mechanism similar to that with CuH-*L-. However, the rate of acid reaction with NiH-*L- is slower than the corresponding reaction with CUHAL-. The addition of a second proton H + + N i L f ) is very rapid in comparison to the reaction with NiH-2L-. The kinetics of reaction of (NiH-,L NiL+ with base to give NiH_,L- are first order in [NiLf] and second order in [OH-]. The NiH-lL complex is thermodynamically unstable relative to NiH-2L- and NiL+. The first and second acid ionization constants of nickel-triglycine are lo-*,*and 10-7.7, respectively. The third acid ionization constant (to give NiH-2L(OH)2-) is 10-12.8. All rate and equilibrium constants are measured at 25.0’ and an ionic strength of 0.16 M .
+
I
onization of peptide hydrogens from blue, octahedral nickel(I1) complexes of triglycine and tetraglycine results in the formation of yellow square-planar complexesa in which the deprotonated nitrogen atoms are coordinated to n i ~ k e l . ~In each complex ionization of the p e p t i d e protons occurs at approximately the same pH. Thus, Kim and Martel14 report that with nickel(11)-triglycine both protons are displaced in a single step in contrast to the stepwise dissociation observed with the copper complex.5 Although ionization of the two peptide protons from NiL+ (where L- is the triglycinate ion) is almost simultaneous, the acid dissociation constants Kal and K,, (eq 1 and 2) are measured in the present work.
Proton transfer to the deprotonated copper(I1)-triglycine complex, CuH-*L-, is much slower than normal diffusion-controlled reactiomF Proton transfer reactions to NiH-?L- are studied and found to be even
slower than to CuH-?L-.
Experimental Section Reagents. Nickel(l1) perchlorate was prepared from nickel carbonate and perchloric acid and was recrystallized from water. A nickel stock solution was prepared and was standardized by EDTA titration using murexide indicator. Solutions of triglycine were prepared by weighing the chromatographicall) pure solid (Mann Research Laboratories, New York, N. Y . )and were used within a few days. Boric acid and sodium perchlorate were recrystallizedfrom water. Solutions of acetic acid and mono- and dihydrogen phosphate
(1) Address correspondence to this author. (2) R . B. Martin, M. Chamberlin, and J. T. Edsall, J . Amer. Chem. Soc., 82,495 (1960). (3) H. C. Freeman, J. M. Guss, and R. L. Sinclair, Chem. Comrnrrn., 485 (1968). (4) M. I
